So what is the theoretical minimum dropout size that we can fit one of these motors into? Dropout distance of 149mm - disc brake side gap of 12mm = axle shoulder to shoulder width of 137mm. I believe the actual spec is 135mm they are machined to, so the difference is just measurement error, but I'll err to the side of being bigger at 137mm.

Add to this the freewheel + washer most likely needed to prevent binding and you end up at 140mm (a thinner washer could be utilized in many cases, I went with a size that I knew would work for all variances that I have encountered in axle offset to prevent binding).

So if you were to run this without a disc brake, you could theoretically run this in a drop out as small as 140mm, but I can't guarantee that due to variances in frame design and small differences between motors. This is why the official spec is 145mm minimum with work, but it can be made to fit smaller dropouts if you have some skill/creativity.